1. Field of the Invention
The present invention relates to a blister-resistant steel sheet applied to automobiles and household electric appliances and to a method for producing thereof. In particular, the present invention relates to a blister-resistant steel sheet using extremely low carbon steel as the body material and a method for producing thereof.
2. Description of the Related Arts
Recent development of the technology of steel making and of degassing allowed the commercial production of extremely low carbon steels containing 50 ppm or less of carbon and nitrogen at a relatively low cost. By adding Ti and Nb, which form carbide and nitride, to the extremely low carbon steel, what is called "IF steel" (carbon and nitrogen are fixed in the steel) was produced. The IF steel was introduced in JP-B-44-18066, JP-B-54-1245, and JP-A-59-673 19 (the terms "JP-B-" and "JP-A-" referred to herein signify "examined Japanese patent publication" and "unexamined Japanese patent publication", respectively). The IF steel is widely used in cold rolled steel sheets which need good formability mainly with respect of deep drawing.
Compared with common low oxygen Al-killed steel, however, that type of IF steel likely generates blisters after the final heat treatment. A blister is a rounded swelling on the surface of metal of an approximate size of 1 mm in width and 10 mm in length. Thicker steel sheet induces blisters more easily. Since a blister bursts open during the processing such as press-forming, it is classified as a surface defect. If any blister is found on the product steel sheet at manufacturer's product inspection, the steel sheet is rejected.
Accordingly, compared with the frequency of blisters generation in manufacture line, the frequency for users to deal with the blister as a serious problem is not necessarily high. As a result, very few papers and patents deal with the blister on IF steel. Nevertheless, for the material manufacturers, the generation of blisters results in a significant decrease of production yield, and particularly when some blisters which could not be found during a user's inspection burst open during the processing such as press-forming, the defect in the processing results in the reduction of productivity.